Exodus 12:7, 12-13

And they shall take of the blood, and put it on the two side-posts and on the lintel, upon the houses wherein they shall eat it.

For I will go through the land of Egypt in that night, and will smite all the first-born in the land of Egypt, both man and beast; and against all the gods of Egypt I will execute judgments: I am Jehovah. And the blood shall be to you for a token upon the houses where ye are: and when I see the blood, I will pass over you, and there shall no plague be upon you to destroy you, when I smite the land of Egypt.